**7. Conclusions**

role both in the prevention of autoimmunity refraining the immune system from attacking cells indiscriminately under normal physiological conditions and in the regulation of immune reaction to avoid tissue damages during the pathogenic infection. Under normal conditions, immune checkpoints function via the interaction between a receptor expressed on T cells and its ligand located at the surface of APCs to generate a co-stimulatory signal, which triggers either the activation or inhibition of T cells. Presently, two major checkpoints have been clearly identified to regulate T cell activation: (i) the CD28/CTLA-4 axis, which activates T cells upon engagement of CD28 with CD80 and CD86, and conversely inhibits T cells when CTLA-4 is engaged and (ii) the PD-1 axis, which provides a strong inhibitory signal following binding of PD-L1 or PD-L2 to the PD-1 receptor [70]. Contrary to CTLA-4, PD-1 is thought to act predominantly in the tumor microenvironment, where PD-L1 is overexpressed by multiple cell types, including dendritic cells, M2 macrophages, and tumor-associated fibroblasts [71]. Thus, immune checkpoints and pathways, unfortunately, are also utilized by cancer cells as a key mechanisms to realize immune escape through upregulated expression of immune checkpoint ligands and as such deregulation of immune checkpoint signaling to suppress T cell activity in tumor microenvironment, a phenomenon that has been observed in multiple malignancies. Moreover, immune checkpoint molecules have been shown to promote the epithelial-mesenchymal transition of tumor cells and the acquisition of tumor-initiating potential and resistance to apoptosis and antitumor drugs, as well as the propensity to disseminate and metastasize, and thus have been increasingly considered as a crucial target for cancer immunotherapy given their potential for use in multiple types of cancers. Notably, as opposed to other immunebased approaches developed to fight cancers, immune checkpoint blockers (ICBs) have displayed significant therapeutic successes in many solid tumors and hematologic malignancies, as exampled by several anti-PD-(L)1-based drugs, such as the anti-CTLA-4 ipilimumab (by Bristol-Myers Squibb), the anti-PD-1 pembrolizumab (by Merck), and the anti-PD-L1 atezolizumab (by Genentech/Roche), durvalumab (by AstraZeneca/MedImmune), and avelumab (by

58 Immunization - Vaccine Adjuvant Delivery System and Strategies

Pfizer), all of which have already been approved for cancer immunotherapy [69].

However, with the current antibody-based immune checkpoint therapy, the nonspecific accumulation of antibody in the normal organs and tissues may ignite overreactive immune responses, which may even damage the body and cause severe side effects [72]; suggesting targeting delivery may provide beneficial effects even in the antibody-based immunotherapy. Recent studies have shown that a diverse set of NPs that have been engineered to improve delivery efficiency of immune checkpoint modulators which possess the potency in enhancement of the anticancer efficacy of the immune checkpoint blockade-based immunotherapy. Using a common procedure of water-in-oil-in-water emulsion, Wang's group formulated cationic NPs loaded with CTLA4 siRNA (siCTLA4) which was to modulate immune suppression mechanism [73]. The siCTLA4-NPs delivered siRNA into the T cells reducing mRNA and protein levels of CTLA4 upon the T cell activation in vitro and, when systemically given to mice, significantly increased the number of both CD4+ T cells and CD8+ T cells, whereas the number of CD4+ FOXP3+ regulatory T cells were decreased, resulting in the inhibited tumor growth and prolonged survival rate of B16 mouse melanoma model. PD-L1 is expressed on a variety of tumor cells, such as melanoma, NSCLC, ovarian cancer, head and neck cancer, B cell lymphoma, and thymic cancer and therefore is another attractive target for immune checkpoint modulation, which can be realized using tumor-targeted delivery system loaded In recent years various types of NPs have been designed as a VADS for delivery of vaccines that are aimed for cancer immunotherapies and have shown great promise in curing refractory tumors which can never be obtained by conventional clinical measures, such as chemotherapeutics, surgery, and radiation. The NP-based cancer VADS possesses numerous advantages, including high safety profile and thus good compliance, high stability, diverse administration routes, and ease in modification with functional molecules as well as large-scale production, and bears also disadvantages including mainly relatively weak immunostimulatory capacity and low intracellular especially intranuclear delivery efficiency, which may be hopefully overcome by elaborate design with adjuvants such as PRRas and multifunctional molecules. Nevertheless, the NP-based cancer VADS proves able to successfully elicit antitumor immunity both in vitro and in vivo through, in particular, targeting APCs and draining lymph nodes, engendering lysosome escape, and modulating immunosuppression and represents new directions in developing efficient tools for cancer immunotherapy.
